The present invention relates to a control system for continuously controlling a driving force distribution between front and rear wheels of a four wheel drive vehicle, and more specifically to a driving force distribution control system which can provide an adequate control performance during deceleration.
Japanese Patent Provisional (unexamined, KOKAI) Publication No. 63-203421 shows a conventional example. A driving force distribution control system of this example is designed to increase an engagement force of a transfer clutch to increase a driving force transmitted to the front wheels in accordance with a front and rear wheel speed difference when a drive wheel slip is produced during acceleration or driving on a slippery road, and varies a gain (rate of increase) of the clutch engagement force in accordance with the lateral acceleration of the vehicle.
This system holds the clutch engagement force equal to zero irrespective of the front and rear wheel speed difference during deceleration during which the rear wheels which are directly connected with the engine becomes lower than the rotational speed of the front wheels which are connected with the engine through the transfer clutch. When, therefore, the vehicle is decelerated during a turn, there is produced a moment tending to turn the vehicle in the steering direction because of a vehicle roll causing a displacement of the center of gravity of the vehicle toward the outside of the turning arc, and a braking force applied on each rear wheel. This system can improve the steering response of the vehicle in yawing motion by producing this moment. However, this system is not designed to control this moment. If this moment increases too much, the possibility of drift-out or tack-in or vehicle spin is increased, and the directional stability of the vehicle becomes poor.